eval: regression in unification with {...} vs evalv2

[myitcv]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.13.0-0.dev.0.20250221114952-15a702eab521

go version go1.24.0
      -buildmode exe
       -compiler gc
  DefaultGODEBUG gotestjsonbuildtext=1,multipathtcp=0,randseednop=0,rsa1024min=0,tlsmlkem=0,x509rsacrt=0,x509usepolicies=0
     CGO_ENABLED 1
          GOARCH arm64
            GOOS linux
         GOARM64 v8.0
             vcs git
    vcs.revision 15a702eab5214e8ee537e74c1ef3c5429a011eca
        vcs.time 2025-02-21T11:49:52Z
    vcs.modified false
cue.lang.version v0.13.0

Does this issue reproduce with the latest release?

Yes

What did you do?

# -- evalv2 --
env CUE_EXPERIMENT=evalv3=0
! exec cue export x.cue
stderr 'z.b: field not allowed'

# -- evalv3 --
env CUE_EXPERIMENT=evalv3=1
env CUE_DEBUG=openinline=0
! exec cue export x.cue
stderr 'z.b: field not allowed'

-- x.cue --
#x: {
	a?: string
}

#y: #x & {
	...
}

z: #y & {
	a: "ok"
	b: "something"
}

What did you expect to see?

Passing test.

What did you see instead?

# -- evalv2 -- (0.009s)
# -- evalv3 -- (0.053s)
> env CUE_EXPERIMENT=evalv3=1
> env CUE_DEBUG=openinline=0
> ! exec cue export x.cue
[stdout]
{
    "z": {
        "a": "ok",
        "b": "something"
    }
}
FAIL: /tmp/testscript3304954446/repro.txtar/script.txtar:9: unexpected command success

This bisects to https://cuelang.org/cl/1204259